Jet breakup is influenced by a range of multi-scale physics; these include the interaction of turbulence with interfaces (creating cascades of motion featuring a large separation of scales), capillarity, potentially complex rheology, the presence of fields (e.g. gravitational, electromagnetic), as well as heat transfer and phase change. [1]} proposed four jet breakup regimes depending on the appearance of the jet far downstream from the injection point. In the `Rayleigh regime', the onset of breakup is the Rayleigh-Plateau instability, in which the growth of the linear modes on the jet-surface leads to the formation of large droplets with respect to the jet-nozzle. In the `first/second wind-induced' regime the resulting droplets have roughly the same scale or smaller as the jet-nozzle. Finally, in the `atomisation regime', the generated turbulence helps to create spatio-temporal chaos resulting in droplets which are up to four orders of magnitude smaller than the size of the injection nozzle. The physics of droplet generation in turbulent jets remains partially-understood despite the significant scientific attention it
has received over the years [2]}, [3]}, [4]}, [5]}, [6]}, [7]}. The rest of this introduction aims to provide an up-to-date summary of the experimental and numerical efforts to study the jet breakup phenomenon.
